How water flows near the superhydrophobic surface

A microsphere attached to the tip of an atomic-force microscope is moving towards the superhydrophobic surface. Due to the unusual texture the viscous response of the water in this system is very different from that of a normal flat wall.
Credit: Taras Molotilin

Water (and other liquids) has an unusual property when it flows closely to some specially designed surfaces: its speed isn‘t equal to zero even in the layer that directly touches the wall. The liquid does not adhere to the surface, but instead slides along it. Such an effect is called hydrodynamic slip. Researchers from Moscow and Aachen now determined this slip length experimentally and examined the analytic formulas that describe the behavior of liquid at different distances from a superhydrophobic surface. Their results may become a starting point for developing new superhydrophobic systems.

Hydrodynamic slip plays an important role for superhydrophobic materials, where chemical hydrophobicity met a peculiar geometry of the surface. In such texture‘s cavities there might be air bubbles trapped, which help the liquid to slip along the surface with almost no resistance, which has greatly increased the slip length in these systems.

Complex superhydrophobic materials required new hydrodynamic theories. Such new approaches predicted not only the decrease of viscous resistance, but also some peculiarities of liquid‘s behaviour near the anisotropic surfaces. For instance, one finds that near the surface covered with elongated grooves that are misaligned to the main flow, the liquid could turn and spin as if by its own. This effect may lead to active mixing or separation of microparticles that are immersed into the flow.

The research team from the Lomonosov Moscow State University, Institute of Physical chemistry and Electrochemistry RAS and DWI have been working for years on theories of hydrodynamic slippage near anisotropic superhydrophobic surfaces.

In the new work, due to the employment of an atomic-force microscope, the scientists have managed not only to determine the slip length experimentally, but also to examine the analytic formulas that described the behavior of liquid at different distances from superhydrophobic surface.